Heat release performance of water-phase change material mixture under periodic heat sink condition

This paper reports the construction of a composite phase change energy storage unit that incorporates water and phase change material (PCM). The presence of water expedited the solidification process of the PCM above it while serving as a sensible heat medium. The influence of heat sink fluctuations on the heat transfer dynamics was investigated during the internal heat release process of the energy storage unit. A comprehensive numerical model was developed, and the Taguchi method was utilized to optimize the design of heat sink parameters. Findings indicated that the contact phase change of water at the unit base significantly accelerated the solidification process of the PCM. Yet, a hard-to-melt region persisted in the upper right corner of the unit at the conclusion of the PCM solidification. Fluctuating heat sinks irregularly impacted water convection at the unit base, although the PCM temperature remained consistent with solidification under a steady heat sink with a given boundary temperature. The influence of sinusoidal heat sink parameters (Foundation T_w, Amplitude (A), half-time period (τ)) was examined using the Taguchi method, revealing that Foundation Tw exerted the most significant influence on solidification time, the average heat release rate of PCM, and the average heat release rate of water, with clear interactions between amplitude and time period. Amplitude and time period affected the sensible heat release process of PCM, while latent heat release remained unaffected by fluctuations due to multiple thermal influences from heat sink, water, and natural convection. Water demonstrated faster heat release compared to PCM owing to its high thermal conductivity, although its fluctuation characteristics were more perceptible. When compared to Case 5 (Foundation T_w = 304 K, A = 7.5 K, τ = 40 s), the solidification time for PCM in Cases 10 (Foundation T_w = 296 K, A = 7.5 K, τ = 50 s) and 11 (Foundation T_w = 296 K, A = 1.5 K, τ = 10 s), was reduced by 27.1% and 27.6%, respectively, with a corresponding increase of 43.1% and 44.4% in average heat release rate of PCM, and a 63.8% and 61.7% increase in the average heat transfer rate of water.